1. Field of the Invention
The invention relates to local area networks and more particularly to the transmission of data between peripheral equipments and core equipments each appropriate for layer 2 switching.
2. Description of the Prior Art
In the present context the expression “layer 2 switching” means switching for transmitting data in point-to-point (“unicast”) mode, point-to-multipoint (“multicast”) mode or multipoint-to-multipoint (“broadcast”) mode. Layer 3 switching concerns routing. Of course, certain equipments, known as switches/routers, provide both layer 2 switching and layer 3 switching.
Many local area networks (LAN) have a physical architecture in which layer 2 switching peripheral equipments are connected to at least two layer 2 switching core equipments. The peripheral equipment is connected by at least one physical link to each of the two core equipments, which are also connected to each other by at least one physical link. This architecture therefore offers redundancy of the links to the peripheral equipments.
However, these redundant links allow the formation of loops that can cause the network to “crash” in the event of transmission of data in broadcast or multicast mode. To remedy this drawback it is advantageous, if not essential, to configure the peripheral and core equipments so that they use a spanning tree technique which consists in detecting and then neutralizing any layer 2 loops. To this end, in the presence of at least two redundant physical links between a peripheral equipment and two core equipments, in the peripheral equipment, one of the two physical ports respectively associated with the two links is placed in a transmission (“forwarding”) mode and the other in a blocking mode for all data transmissions. Thus the data broadcast can take only the path that includes the port that is not blocked.
Because the links containing blocked ports are not used, some bandwidth is wasted. Moreover, when the link containing a port that is not blocked or the link between core equipments is interrupted, it takes a relatively long time, up to 30 seconds, for this technique to unblock another link containing a port that was previously blocked. A faster technique known as “rapid reconfiguration” has been proposed, defined by the IEEE standard 802.1D-2004, and allows link unblocking in less than one second, but as a general rule there are always links including a blocked port and thus some bandwidth is always wasted, which is the payback for providing redundancy.
To improve bandwidth use, the IEEE 802.3ad standard has been proposed, whereby the redundant links are aggregated to offer the peripheral equipment a greater data transmission capacity. This enables the implementation of a load sharing mechanism and a fast mechanism for re-establishing links in the event of an equipment failure. However, this standard requires that the aggregated redundant links terminate at the same core switch, which limits its benefit. Furthermore, this standard does not provide a solution to the problem previously referred to resulting from using the spanning tree technique.
Nortel has proposed the “Split-Multi-Link Trunking” (S-MLT) technique for aggregating links outgoing from the same peripheral equipment and terminating at two different core equipments. However, the peripheral equipment is not informed of a problem on the link between the two core equipments, with the result that some of the data transmitted using the aggregated links may be lost. Thus the integrity of the local area network is not maintained. Moreover, this is a proprietary solution. Furthermore, this technique imposes a volume of traffic between the core equipments for them to be able to synchronize their knowledge.
Hewlett Packard has proposed the “switch meshing” technique as an alternative to the spanning tree technique, enabling improved use of bandwidth. The redundant links can be used to transmit data at all times, allowing a rapid response in the event of a problem on a link. This technique is beneficial from the traffic engineering point of view, but necessitates specific processing at each equipment. Moreover, because of the complexity that it introduces into the local area networks, this technique is not readily compatible with their growth. Furthermore, it is a proprietary solution for all the equipments (peripheral equipments or core equipments). Additionally, the number of peripheral and core equipments that can offer the function is limited to 15.
Because there is no prior art technique that is entirely satisfactory, an object of the invention is to improve on the situation.